A selective-reduction catalyst system using a selective-reduction catalyst has been developed as an exhaust gas aftertreatment system for purifying NOx in an exhaust gas of a diesel engine.
This selective-reduction catalyst system provides urea water accumulated in a urea tank to the upstream of exhaust gas with respect to the selective-reduction catalyst, and generates ammonia with the heat of the exhaust gas, and reduces NOx with the selective-reduction catalyst using this ammonia and purifies the exhaust gas (for example, see Patent Document 1). The urea water is injected from a dosing valve provided at the upstream side with respect to the selective-reduction catalyst, so that the urea water is provided to the upstream of the exhaust gas with respect to the selective-reduction catalyst.
The urea water is provided to the dosing valve by using a supply module (SM) having a supply module pump (SM pump), a urea water pressure sensor, and the like. The supply module is connected via a liquid feeding line to the urea tank, and provides the urea water, which has been sucked from the urea tank via the liquid feeding line, to the dosing valve via a compression feeding line connecting the supply module and the dosing valve. At this occasion, a measurement value of a urea water pressure sensor of the supply module is feedback controlled so as to attain a constant level, and when the urea water pressure becomes constant, the urea water is started to be injected.